1. Field of the Invention
This invention relates to an optical fibre coupling device.
2. Description of Related Art
In the coupling of laser light into a single-mode optical fibre it is important that losses in the transmitted light intensity should be kept low so that the total losses in a single-mode transmission system can be minimised. The launching of the light is usually effected from a single mode waveguide device such as a laser or amplifier and, whilst a perfect launching arrangement is theoretically possible, practical constraints usually limit the coupling efficiency to only about fifty percent, that is with a loss of more than three decibels (dB). Many real systems in practice will show losses as high as 5 dB.
Attempts have been made to improve the coupling efficiency by using lensing schemes which are intended to match the typically small and elliptically-shaped laser mode to a larger spot area requirement and a circular fibre mode. Thus, chemically etched conical microlenses have been disclosed in a paper by G. Eisenstein and D. Vitello in Applied Optics, Vol. 21, No. 19, Oct. 1, 1982, pp 3470 to 3474. However, these lens devices can still give high losses and the conical external shape may represent an unwanted feature. Losses are mainly due to the effect of a partial cancellation between different parts of the launch wave due to the existence of a phase mismatch in the wave.
This source of losses is capable of being corrected and the present invention was devised as a way of making this correction which enables certain difficulties in implementation to be avoided.